Cokes are used as blast-furnace raw materials to produce hot metal. High strength cokes are desirable because otherwise the cokes are disintegrated in the blast furnace to deteriorate gas permeability in the blast furnace and to destabilize the production of hot metal.
When metallurgical coke is produced by carbonizing coals in a horizontal chamber coke oven, the strength of the metallurgical coke is variable depending on factors such as the selection of raw material coals, preparation methods, carbonization conditions, quenching conditions and post-treatment conditions. Of these factors, the conditions associated with the facility and the operation conditions are facility restrictions and thus cannot be easily altered to a great degree. Thus, the selection of raw material coals is considered as the most important factor for controlling coke properties.
Various coal blending methods for obtaining cokes having desired strength are known in the art, and one such method is mentioned in Non Patent Literature 1. All these methods determine suitable blending by predicting the strength of coke produced based on the properties of raw material coals that are blended.
However, such known methods are at times incapable of accurately predicting the coke strength. One possible reason is a phenomenon called the “compatibility of coals”. Non Patent Literature 2 mentions that there is often no additivity between the strengths of cokes obtained from individual brands of coals that are to be blended together, and the strength of a coke obtained from a coal blend including a plurality of brands with different properties. The “compatibility of coals” may be represented by the difference between a theoretical sum and a measured value of coke strength. When the measured value is larger than the theoretical sum, the compatibility is “good”. The compatibility is “bad” when the measured value is smaller than the theoretical sum. Various studies have been carried out to determine the cause that gives rise to the “compatibility” effect. However, no techniques have been established that evaluate the “compatibility of coals” and specify a combination of good compatible coals.
The compatibility of coals is probably affected by variable adhesion strengths between different kinds of coals, and various techniques have been studied for the evaluation of adhesive strength at interfaces of different kinds of coals. For example, interface contacts between various combinations of different coals are observed and classified into four contact conditions, namely, diffusive contact, non-diffusive contact, fissile contact and porous contact. Based on the assumption that these conditions are brought about by the caking properties of coals and also by micro fissures generated by shrinkage when the coals are melted by heating and then become solidified, the adhesiveness is evaluated based on the maximum fluidity obtained by the Gieseler plastometer method that is a caking index, and on the solidification temperature. (See Non Patent Literatures 3 and 4.)